1. Field of the Invention
The invention relates generally to a liquid crystal lens that is capable of adjusting a liquid crystal lens quality, being controlled by thin film transistors (TFTs), and having a high transmittance, and more particularly, to a double-layer liquid crystal lens apparatus.
2. Description of Related Art
Over the years, a number of publications have disclosed techniques relating to liquid crystal optical devices which employ liquid crystal materials having special optical characteristics with adjustable refractive indices when external voltages are applied thereto, so as to alter a focal length of a lens for applications in various fields. The concept of using liquid crystals to fabricate refractive optical devices is first disclosed in U.S. Pat. No. 4,066,334, which describes applying an external voltage to adjust a rotational direction of the liquid crystal molecules so as to vary a refractive index, and thereby causing an incident beam to be deflected in a liquid crystal material. Afterwards, there have been many publications which describe liquid crystal optical devices utilizing the adjustable characteristics of the liquid crystals. For example, in an electrode design according to U.S. Pat. No. 6,577,376 for driving liquid crystals, diffraction zone plates designed as concentric circles are used to form a diffractive liquid crystal device, so as to generate a first-order diffraction beam and a zero-order non-diffractive beam for an incident beam of a predetermined polarization direction. The device is complemented with a focusing lens to generate two focus points of different positions, for application in an optical pickup head to read/write multi-layered data, as well as for application in an aberration correction unit.
Moreover, in U.S. Pat. No. 6,690,500, the driving electrodes of a liquid crystal device are cyclic ring-shaped structures designed as concentric circles. Each of the ring-shaped electrodes is driven with different high and low voltages for continuous voltage drops, so that the refractive indices of the liquid crystal molecules when the electrodes are operating are continuously distributed. Accordingly, by phase adjusting the optical path differences to form structures such as a Fresnel lens, the device can be applied in the aberration correction of the optical pickup head focus points. Further, as disclosed in U.S. Pat. No. 7,262,820, an electrode design therein has two matching upper and lower semi-circular regions, for application in the aberration correction of the optical pickup head, and especially for correcting coma aberration due to tilt.
In addition, U.S. Pat. No. 6,864,951 describes combining the use of an inhomogeneous polymer dispersed liquid crystal (PDLC) combined with an ultraviolent (UV) light radiation, so that the liquid crystal molecules form droplets of uneven size, and an optical focusing property can be continuously modulated by changing an externally applied voltage. Further, as disclosed in an international scientific journal APPLIED OPTICS (Vol. 43, No. 35, p. 6407, December, 2004), the electrodes are configured on another side of a glass that is far thicker than a liquid crystal medium layer. By applying a substantially higher voltage, after an electric potential distributes and passes through the glass, the electric potential has a distribution of continuous curved surfaces, thereby resulting in a continuous liquid crystal refractive index distribution analogous to a lens having a continuous phase distribution, along with a superior focusing property and a simple device structure. However, a driving voltage required is as high as 100 V, far more than the driving voltage of about 5 V for a typical liquid crystal device.
The aforementioned conventional liquid crystal optical devices such as those disclosed by U.S. Pat. Nos. 6,577,376 and 6,690,500 adopt strip-shape or Fresnel lens diffractive device designs. However, due to the diffraction orders generated through diffraction, an overall usable efficiency of a light beam is lowered. On the other hand, because of the restrictive complexities in an UV light exposure process, for the disclosure of U.S. Pat. No. 6,864,951 adopting the PDLC structure, a plurality of restrictions are placed on the medium layer due to factors such as a low transmittance from dispersive effects and a high driving voltage requirement. Most importantly, because of a birefringence property of liquid crystals, the adjustable optical characteristic thereof is applicable only for incident beams of a predetermined polarization, and thus its range of applications is somewhat limited. The aforementioned conventional techniques are applicable only in optical systems using a laser light source or those including a polarizer and an analyzer, since these techniques cannot be effectively employed for imaging systems adopting a typical light source.
Moreover, as disclosed in U.S. Pat. No. 6,859,333, a liquid crystal lens adopting a double-layer liquid crystal layer structure attempts to solve an issue of polarization selectivity of the liquid crystal device. However, the disclosed invention adopts a spherical electrode substrate along with an evenly distributed liquid crystal medium, wherein an adjustable focal length is achieved by applying a voltage to drive the liquid crystals so as to compensate for a fixed curvature of the spherical electrode. The fabrication of the spherical electrode is a main difficulty for this disclosed invention. Furthermore, in a disclosure published in an international scientific journal OPTICS EXPRESS (Vol. 15, No. 6, p. 2900, March, 2007), a photo-alignment method is employed to realize a liquid crystal lens that is independent of light polarization directions. However, the liquid crystal lens has a complicated fabrication process, which remains an important factor to consider.